The present invention relates to a method of producing a porous material having open pores. More particularly, the present invention pertains to a method of producing a porous material having open pores from a mixture which comprises (i) an epoxy compound having at least one epoxy ring in the compound, (ii) a hardener which can react with the epoxy compound to cause hardening thereof, (iii) a filler and (iv) water to which soluble salts are further added.
Hitherto, there have been proposed a variety of methods for producing porous materials having open pores and used as a filtering medium, air diffusion medium, casting mold and so forth; for instance, sintering of metal powder, sintering of thermoplastic resin powder, sintering of inorganic powder, hydration setting of cement or the like, press molding or stamping of a mixture of a thermoplastic and a filler, hardening of a resin solution comprising a pore-forming agent followed by the removal of the pore-forming agent through dissolution, extraction or evaporation, use of a foaming agent, polymerization for hardening a W/0 emulsion such as a water-containing polyester resin followed by evaporation of the water from the hardened mass.
However, these known methods of producing porous materials having open pores inevitably suffer from one or more of problems concerning molding, such as that the products prepared by these methods are considerably limited as to their shapes and sizes, that these methods require the use of a heat treatment at a high temperature and/or high-pressure pressing and that the manufacturing processes are very complicated.
Moreover, these known methods have another problem in that it is very difficult to precisely control the pore size of the product. The control of the pore size is the most important factor in the production of porous materials having open pores used as filtering mediums and air diffusion mediums.
As a method of producing porous materials having open pores which can solve the foregoing problems and which can provide a large scale porous material having open pores, a complicated shape and a desired pore size with good dimensional accuracy, there has been known a method which comprises preparing an emulsion slurry by mixing an epoxy resin, a hardening agent, a filler and water and stirring the mixture to obtain an emulsion slurry, hardening the resulting slurry while maintaining the water content thereof and then removing the water from the hardened mass to form open pores therein. For instance, Japanese Examined Patent Publication (hereinafter referred to as "J.P. KOKOKU") No. 2464/1978 achieves a desired object by preparing an O/W type emulsion slurry from a mixture comprising a glycidyl type epoxy resin, a polymeric fatty acid polyamide hardener, a filler and water, casting the slurry in a water-impermeable mold, hardening the slurry while maintaining the water content thereof and then dehydrating the hardened mass. This conventional method makes it possible to form a large scale porous material having open pores and a complicated shape with good dimensional accuracy and to precisely control the pore size thereof by adjusting the particle size of the filler, the amount of a reactive diluent and the relative compounding ratio among the epoxy resin, hardener, filler and water, but the pore size of the porous material prepared by this method is very small of the order of not more than 1.5 .mu.m and, therefore, the material is impracticable for use as filtering mediums, air diffusion mediums and casting molds.
This problem can be solved by a method of producing a porous material having open pores disclosed in J.P. KOKOKU No. 26657/1987, which comprises preparing a hardener by admixing an amide compound obtained through the reaction of a monomeric fatty acid with an ethyleneamine: H.sub.2 N--(CH.sub.2 --CH.sub.2 --NH).sub.n --H (wherein n is an integer ranging from 3 to 5) and a polymerized fatty acid polyamide obtained through the reaction of a polymerized fatty acid with the foregoing ethyleneamine, or by mixing and reacting the monomeric fatty acid, the polymerized fatty acid and the ethyleneamine to obtain a mixed reaction product, vigorously stirring a mixture comprising a bisphenol type epoxy resin, the hardener, a filler and water to obtain an emulsion slurry, casting the slurry in a water impermeable mold, hardening the slurry while maintaining the water content thereof and then dehydrating the hardened mass. This method permits the formation of a large scale porous material having open pores, whose average pore size ranges from 0.5 to 10 .mu.m, preferably 0.5 to 5 .mu.m, and in particular 1.5 to 5 .mu.m with good dimensional accuracy. In addition, Japanese Unexamined Patent Publication (hereinafter referred to as "J.P. KOKAI") No. 75044/1988 likewise proposes a method of producing a porous material having open pores and a pore size ranging from 0.2 to 10 .mu.m which comprises preparing an emulsion slurry from a mixture containing a glycidyl type epoxy resin, a polyamide hardener and a modified polyamide hardener and/or an amine hardener, a filler, and water, and then casting the slurry in a water impermeable mold, hardening the slurry while maintaining the water content thereof and then dehydrating the hardened mass.
However, though these methods permit control of the pore size of the resulting porous material, they cannot provide porous materials having open pores having a relation between the pore size and the air and water permeabilities which is always kept unchanged. More specifically, the water and air permeabilities are sometimes insufficient even if the pore size is satisfactorily large. This uniformity in the relation between the pore size and air and water permeabilities of the porous material having open pores is very important when the porous material is used as filtering mediums, air diffusion mediums and casting molds used in industry. Also, control of the water and air permeabilities of porous materials is very important in industrial applications thereof, but it has been impossible to control them according to the conventional techniques.
Furthermore, it is necessary to eliminate the scattering in shrinkage of porous material encountered during hardening of the same if high dimensional accuracy is required, but the conventional techniques cannot solve this problem at all.